In the decade of the 1950s, predictions were made of an impending global protein shortage. Interest developed in the potential of microbiologically-produced protein as an animal feed supplement, and, possibly, as a human dietary constituent.
Yeasts are known for a high nutritional value, and have been used for a long time as feed supplements, both for animals, and to some extent for humans. Yeasts have a high protein content in general, and probably contain more B-group vitamins than most other protein sources, B.sub.12 a notable exception. However, yeasts are also known to be low in some of the sulfur-containing amino acids, particularly methionine.
TABLE I.sup.a __________________________________________________________________________ ESSENTIAL AMINO ACIDS OF SOME PROTEIN SOURCES.sup.b Saccharomyces Candida Pichia pastoris Wheat Egg White Cerevisiae Lypolytica NRRL Y-11430.sup.c __________________________________________________________________________ Lysine 2.8 6.5 7.7 7.8 6.1 Threonine 2.9 5.1 4.8 5.4 5.6 Methionine 1.5 3.2 1.7 1.6 1.2 Cystine 2.5 2.4 -- 0.9 0.7 Tryptophan 1.1 1.6 1.0 1.3 0.7 Isoleucine 3.3 6.7 4.6 5.3 5.5 Leucine 6.7 8.9 7.0 7.8 7.8 Valine 4.4 7.3 5.3 5.8 6.5 Phenylalanine 4.5 5.8 4.1 4.8 4.1 __________________________________________________________________________ .sup.a Data from p. 110 Industrial Applications of Microbiology, J. Riviere, (Wiley 1977). .sup.b The values shown are g amino acid/100 g protein. .sup.c Methionine content in Pichia pastoris Y11430 is 1.2 g/100 g protei (as shown) which equals 0.8 g/100 g weight.
TABLE II.sup.a ______________________________________ LIMITING AMINO ACIDS OF COMMON FOOD PROTEIN Food Source Limiting Amino Acids % Deficiency.sup.b ______________________________________ Whole Egg none 0 Beef Cystine and Methionine 29 Cow's Milk Cystine and Methionine 32 Rice, Wheat, Maize Lysine 61-72 Baker's Yeast Cystine and Methionine 55 ______________________________________ .sup.a p. 112 of reference cited Table I footnote a. .sup.b Compared with whole egg.
As can be seen above, added methionine is needed when yeasts are to be used as animal feeds.
Needed are suitable yeasts that have good productivity in single cell protein production, but with much increased (enhanced) methionine content.